1. Field of the Invention
The present invention broadly relates to energy-trapping thickness-shear resonators and, more particularly, to a structure of a miniaturized energy-trapping thickness-shear resonator.
2. Description of the Related Art
Along with the miniaturization of electronic equipment, in particular, mobile communication equipment, there is an increasing demand for further reducing the size of power supply devices, such as batteries and secondary cells. Moreover, lower-powered oscillators are also required to enable electronic equipment to be used for as long as possible. However, it is difficult to effectively decrease the power consumed in currently available ceramic oscillators because such oscillators tend to be operated at higher frequencies and the inter-terminal capacitance or the load capacitance is comparatively large.
As a type of oscillator described above, an energy-trapping thickness-shear resonator having the structure shown in FIG. 1 is widely used. The resonator A is formed of a thin rectangular piezoelectric substrate 1. In the resonator A shown in FIG. 1, an electrode 2 is arranged to extend from one end to a central portion on one surface of the substrate 1. Another electrode 3 is also arranged to extend from the other end to the central portion of the other surface of the substrate 1. Thus, the electrodes 2 and 3 opposedly face each other at the central portion of the piezoelectric substrate 1, thereby causing thickness shear vibrations.
The inter-terminal capacitance C.sub.f of the resonator A having the structure and arrangement as described above is expressed by the following equation: EQU C.sub.f =.epsilon..sub.0 .epsilon..sub.r .DELTA.LW/T
where .epsilon..sub.0 indicates the vacuum dielectric constant; .epsilon..sub.r represents the relative dielectric constant of the constituent material for the piezoelectric substrate; .DELTA.L designates the length of the overlapping portion of the electrodes 2 and 3; W indicates the width of the piezoelectric substrate 1; and T represents the thickness of the substrate 1.
In the above equation, in order to reduce the capacitance C.sub.f, among the elements which determine the capacitance C.sub.f, the length .DELTA.L or the width W is decreased or the thickness T is increased. The thickness T however cannot be easily changed because there is a certain correlation between the thickness T and the oscillation frequency. It is not easy to change the length .DELTA.L, either, because a substantially fixed ratio .DELTA.L/T is, in general, required for preventing spurious responses from being output from the oscillator. Accordingly, the width W can be most effectively decreased to reduce the capacitance C.sub.f.
In known oscillators utilizing thickness shear vibrations as described above, the range of the width W is determined in the following manner. For example, Japanese Unexamined Patent Publication No. 63-206018 discloses an oscillator having a width W ranging of 0.8 mm.ltoreq.W.ltoreq.1.4 mm. Also, Japanese Unexamined Utility Model Publication No. 1-171121 discloses an oscillator having a width W ranging of 0.75 mm.ltoreq.W.ltoreq.0.90 mm.
However, a large amount of power is still needed even in the oscillators having the ranges of widths W determined in the above manner. Accordingly, it is necessary that the width W be decreased even more. Through a variety of tests concerning the relationship between the width and power used in the oscillators utilizing thickness shear vibrations, the present inventors have discovered a relationship between the width and power as shown in FIG. 2. It should be noted that the data indicated in FIG. 2 was obtained with the condition that .DELTA.L/T was substantially fixed.
FIG. 2 reveals that power consumption starts to sharply decrease as the width W becomes equivalent to or smaller than approximately 0.7 mm. In the above-described ranges of the width W, however, spurious vibrations may be generated in the frequency band depending on the value of the width W. The spurious vibrations cause abnormal oscillations, such as the case where the oscillation frequency is carried on the frequency of the third-order or fifth-order harmonics of the shear vibrations. In particular, there is a greater likelihood that larger spurious responses are generated in oscillators operated at higher frequencies, thereby preventing such oscillators from being put into practical use.